When a well such as an oil or gas well has been drilled, it is often desired to isolate the various producing zones from each other or from the well itself in order to stabilize the well or prevent fluid communication between the zones or shut off unwanted fluid production such as water. This isolation is typically achieved by installing a tubular casing in the well and filling the annulus between the outside of the casing and the wall of the well (the formation) with cement. The cement is usually placed in the annulus by pumping slurry of the cement down the casing such that it exits at the bottom of the well and passes back up the outside of the casing to fill the annulus. Various additives can be added to the cement to enhance properties of the slurry or of the set cement, for example fluid-loss control agent, foaming agent, dispersant, retarder, accelerator . . . .
Hydroxyethyl celluloses (HEC) of different molecular weights (Mw) are currently used in oilwell cementing. By way of examples, a low molecular-weight HEC, is used as a fluid-loss control agent in cement slurries having a Solid Volume Fraction (SVF) of about 35% to about 60%. A low/medium molecular-weight HEC is used as a fluid-loss control agent in cement slurries having a SVF of about 20% to about 30% (i.e., extended slurries). And a high molecular-weight HEC, is primarily used as an extender in extended cement slurries. Last one also provides some fluid-loss control.
As well, AMPS-Acrylamide copolymer is used as a fluid-loss control agent in cement slurries having a SVF of about 35% to about 60% (U.S. Pat. No. 6,277,900). It is not frequently used in extended slurries since it does not control properly the fluid-loss rate and does not impart high slurry viscosity.
For instance, the density of cement slurries can be decreased by adding large quantities of water to the cement. An extender has to be added to prevent both the settling of cement particles and the appearance of high amounts of free fluid. Typical extenders are sodium silicates, clays (e.g., bentonite), silica fume and high molecular-weight water-soluble polymers (e.g., polysaccharides). Among these extenders sodium silicates are commonly used. The addition of a retarder is often necessary when sodium silicates, which significantly increase the hydration rate of cement, are used as extender. In the presence of retarder the development of high amounts of free fluid with hydration time can be a major issue. A water-soluble polymer can be used in conjunction with the sodium silicate to stabilize the cement slurry and to decrease the amount of free fluid. A hydroxyethyl cellulose (HEC) of low/medium molecular weight is often used. In addition, this HEC enables to control the fluid-loss rate and to increase the rheology of cement slurry. It is reminded that the viscosity of cement slurry must be high enough to remove properly the drilling mud that has to be displaced by the cement slurry. Unfortunately in many cases use of HEC of low/medium molecular weight does not allow to obtain a satisfactory compromise between the slurry rheology (measured after cement mixing and after slurry conditioning at test temperature), the slurry stability (cement particles settling and free fluid) and the fluid-loss rate.
Therefore, a solution to decouple the fluid-loss control and the slurry rheology is needed for cement slurry and particularly for extended cement slurry.